All parasite treatments are basically poisons, that are too week to directly kill fish in small doses, but add them constantly and they will build up to toxic conditions enough to stress out, and eventually kill your fish. As stated above, garlic is a good preventative for internal parasites, and UV can kill free floating ones. Salt helps a little, but many parasites are halophyllic (salt tolerant).
Best practices are the way to go, ie quarantining new fish, plenty of water changes with substrate vacuuming, keeping stress down, (not over crowding, not overfeeding, keeping compatible fish together, good water quality etc etc) and only using feeder fish that you breed yourself, (and only if needed for piscivores that absolutely won't eat prepared foods).